By the Light of the Moon

My daughter just wrote to ask me just who can see the moon at any given moment in time.  If she can see it, can people in other countries see it too?  I wrote her a fun answer, and I'll share it here...

It's fascinating to picture how the moon works; what parts are lit, where it can be seen from, and so on.  The best thing to do is to imagine, or actually hold, two balls in front of you, like a golf ball and a grape perhaps. Hold them with your hands out a foot or so from you and from each other. Now imagine the sun as a big beach ball across the street. Effectively, the sun is "infinitely" far away, so the light always shines on the same side of the two balls you're holding, with all sun beams pretty much perfectly parallel. As you move the balls around each other you can see how the lighting on each changes relative to each other. This can help you visualize the phase of the moon as seen from the earth, and how the moon is visible from almost a full half of the earth at all times.

One cool detail I figured out one day is that if you look at the moon in the sky and imagine yourself standing up around the top edge, as though gravity there was pulling in the same direction as where you're standing on the earth, some interesting information comes to light. Another way to say this is to imagine a flag pole on top of the moon, sticking straight up from the moon's surface, exactly parallel and pointing in the same direction as a flag pole you're standing next to... okay, now look at where the sunlight is on the whole sphere of the moon around and under that flagpole. Is the flagpole on top up in the light or in the dark?  Then realize that the light on the whole earth under your feet is exactly the same. This is because the sun is so far away and both the earth and moon are lit in "parallel", or at exactly the same angles.  Looking at the moon in this way you can visually imagine how the earth is currently lit up too.  If the flag pole up around the top corner of the moon looks like it's in the dark, then you're probably looking at the moon in the dark too.

As far as who can see the moon at any given momnent, reverse the situation and imagine you're standing on the moon. From there you can see almost half of the earth at all times.  Anyone standing on the earth where you could see their location can look up and see your location on the moon too. This "line of sight" explanation can help you imagine where the moon must be visible from at any given moment.

A globe of the earth can help with this visualization. Find your location on the globe of the earth, put your eyes out away from the globe at about the angle the moon is up in the sky, and then see what half of the globe you can see.  Be sure to hold the globe far enough away from you to be able to see almost a full half of the earth. The moon is far enough away from the earth that almost (but not quite) a full half of each is visible from the other.

* * *

Astronomical algorithms are a favorite subject on mine. This past month I republished a 2012 edition of Sun Position, my Kindle book that presents two very high accuracy algorithms for calculating exactly where in the sky the sun can be found... invaluable for solar energy research, development, and applications.

"Dark Side of the Moon"

Today the space probes Ebb and Flo crashed into the mile-high edge of a crater on the moon after an extremely successful mission to map the subtle gravity changes all over the moon's surface. Yesterday, one well-meaning news reporter stated the lunar satellites were to crash on the "dark side of the moon", and man, did he get a bunch of people writing to set him straight!  

You see, supposedly there really is no dark side of the moon, because the sun shines on all parts of the moon over a one-month period. This makes sense, but I'm here to tell you there really IS a dark side of the moon after all, and here's why.

Standing on the moon looking up towards earth, you'd discover that the phase of the earth is exactly opposite the current phase of the moon. If the moon is full, the earth is barely lit (a "finger nail earth"), and when the moon is but a fingernail of light, the earth is full and very bright in the lunar sky.  This is all because of the way the earth-moon-sun geometry all works out, with the sun at a very long distance away from the relatively close-together earth and moon system.

This all means that from the perspective of someone standing on the near side of the moon, the sun is very bright when its up for half a month, and the earth is very bright in the sky for the other half of the month.  If you're on the near side of the moon, the earth always hangs up there pretty much in the same spot in the sky, always looming large, especially when the sun is down and the earth is all lit up. 

Now consider the view of the sky for a lunar explorer standing on the far side of the moon, where the earth is never visible. Half of the month the sun is very, very bright, as before, but for the other half of the month the sky is awesomely dark, with the stars, milky way, and a few planets brilliant and unblinking in the inky blackness.  

In summary, one side of the moon is always bathed in the glow of the sun or the earth-shine, and the other side of the moon spends half of each month in inky blackness.  The far side of the moon truly can be considered the "dark side of the moon".

* * *

Astronomical algorithms are a favorite subject on mine. This past month I republished a 2012 edition of Sun Position, my Kindle book that presents two very high accuracy algorithms for calculating exactly where in the sky the sun can be found.